Dear Chang,
Typical Modern microprocessors take about 45 -65 W/cm^2 in today's technologies. Refer to Intel's Power group which has all the facts on their websites. Another good reference is **broken link removed**
That was the reason why I said that your circuit is a hot bulb.
May be you must have computed by the equation power density = V*I/area of the transistor., where V is the power supply voltage and I the rms current. You should also try to factor in the duty cycle of switching in case of a switch or the 2*Vdsat of the the device, if it is operating in saturation.
I am sure that your pass transistors or power transistors in a voltage regulator will have higher power densities. Hence they are laid out at the periphery of the chip to alleviate thermal issues and IR drops.
I hope this helps